Failure of a skeletal joint in any animal can be a crippling or even fatal occurrence. Joint failure can result from disease, trauma or wear. To compensate for improper functioning of a joint, animals will often change their behavior, including posture and pattern of movement. These adaptations enjoy limited success, and often reduce loads on damaged joints only to distribute them, inappropriately, to other skeletal and muscular components. This deleterious compensation often leads to secondary, traumatic failures at other vulnerable locations.
Significant developments in the field of joint prosthetics for transplantation have been primarily limited to the second half of this century. Despite substantial advances in surgical techniques, prosthetic materials, and therapies, the life expectancy of artificial joints remains limited. In general, life expectancy depends upon the complexity of the joint repair or replacement surgery, the design of the prosthetic joint device, and the age and weight of the patient. Average hip transplants, for example, will last between 8 to 12 years. The life expectancy of artificial knee joints are even more restricted.
In addition to the poor life expectancy of current artificial joints, the surgery which is required to implant joint prosthetics is particularly invasive. For example, hip replacement surgery often requires a hospital stay of up to two weeks, and several months of rehabilitation. Costs can run in excess of $ 40,000. Thus, in view of both the poor life expectancy, high costs and patient morbidity, joint replacement is generally not performed until it becomes unavoidable. Delaying replacement in this manner causes further damage to otherwise healthy tissues, and unnecessary pain to patients.
To date, a wide variety of artificial replacements for ball and socket joints have been developed. For human hip replacement, several closely related designs have become standards in the industry. These prosthetic devices follow the same basic principles in both design and implementation. In the traditional hip replacement procedure, an artificial socket is first embedded in the patient's acetabulum. The acetabulum is the convergence of the ilium, the ischium and the pubis; a naturally occurring cup or socket that accommodates the femoral head, a 3/4 sphere covered with thick tenacious articular cartilage. The replacement ball component, if required, is then attached to the femur and inserted into the artificial socket. In some cases it is possible to retain the patient's natural femoral head.
To retain the ball in the socket, a number of different designs have been developed. In the most common method, referred to herein as a "semi constrained" construction, the patient's muscles, tendons and ligaments, are used to retain the ball within the socket. In these designs, a hemispherical socket is used which accommodates the ball and allows the attached femur a wide range of movement. When installing a semi constrained joint, aligning the ball and socket as closely as possible with the patient's natural anatomy is of key importance. This is to ensure that the patient's movements do not dislocate the ball from the joint.
To increase the stability and avoid post-operative dislocations in such semi-constrained constructions, a cylindrical portion is added to the hemispherical socket to make it deeper. The ball is not physically constrained by the socket, but it does have further to travel than if just a hemisphere is used. Ball and socket joints of this type generally provide an arc or range of motion of approximately 1150.
Dislocation frequencies of less than 5% for hip joint implant procedures using a semi-constrained construction have been reported. However, even a low frequency of dislocations is significant, because dislocation can render the patient immobile and can require a second operation. In this event, the critical alignment required for a semi-constrained construction is even more difficult to achieve. Thus, higher dislocation frequencies are encountered in the case of sequential implantations.
An alternative to the semi-constrained construction is a construction wherein the ball is physically constrained within the socket. In this construction, a spherically shaped bearing surrounds the ball and serves as the socket. The bearing encompasses more than one-half of the ball and thus constrains the ball and femoral component from dislocation. The slide bearings in these artificial joints are typically made from plastics, such as high density polyethylene or metal. The more the bearing encompasses the ball, the smaller the range of motion for the femoral component prior to contact with the bearing. For these constructions, it has been found that a dislocating force is created when the neck of the arm attached to the ball impinges on the rim of the bearing. Because of the leverage associated with the length of the femur the dislocating force produced when the femoral component contacts the rim of the bearing can be substantial. A relatively small force applied to a patient's leg can thus produce a dislocating force of over several hundred pounds, due to the substantial leverages involved. Dislocation forces must therefore be avoided in the same way as dislocations are avoided in the semi-constrained construction, i.e., through precise alignment of the artificial joint with the natural anatomy of the patient.
Artificial hips having this type of constrained architecture have been found to suffer dislocation due to leverage in fewer than 0.5% of the implantations performed. This is significantly better than the dislocation frequency reported for semi-constrained implants, but an even lower dislocation frequency is of course desirable.
In addition to the risks of dislocation, another problem with artificial joint replacement is attributed to deterioration of the prosthetic components. For example, plastic bearings wear out over time. Accompanying this wear, friction and stresses resulting from post-operative use of the joint typically produce particulates which accumulate and hasten failure of the joint. This deterioration often results in substantial pain and damage to the surrounding tissue. In general, deterioration of surrounding tissue, particularly bone mass, which accompanies artificial joint wear renders subsequent implantations more difficult and less likely to yield a successful result.
A fundamental design problem with existing artificial joints is that prosthetic engineers have attempted to recreate, using man-made materials, replicas of naturally occurring joints. Unfortunately, man-made materials are not yet the equal of their natural, living counterparts. They do not do the job as well and they break down more quickly due to the lack of self-repair capacity.
It is therefore an object of the present invention to provide an artificial joint which supplies the patient with a sturdy, durable replacement prosthesis by reducing frictional forces and increasing the strength of the joint so that it will last a patient's life time.
Another object of the invention is to provide an artificial joint which resists post-operative dislocations without sacrificing range of motion.
A further object of the invention is to provide an artificial joint which delivers increased shock absorption and transmission, resulting in more comfortable use by the patient.
Yet another object of the present invention is to provide an artificial joint which allows surgeons a greater degree of latitude in geometric positioning of the joint during surgical implantation than can be achieved with presently available joint prostheses.